In many diagnostic methods, cells are taken from a subject and deposited into a liquid medium containing a fixative. The cells are fixed in the medium and examined cytologically in order to provide a diagnosis. For example, detection of precancerous or cancerous cells in cervical tissue is routinely performed by microscopic assessment of exfoliated cervical cells. This method, developed by George N. Papanicolaou and known as the “Pap” test, involves exfoliating cells from a woman's cervix using a sampling device, depositing the exfoliated cells into a transport medium that contains a fixative, and then depositing the cells onto a slide. The cells are then stained and examined by light microscopy for cellular abnormalities by a trained medical professional. Over 55 million Pap tests are performed each year in the United States alone.
Despite the success of such cytological tests, the tests are prone to error. For example, it has been estimated that up to 40% of conventional Pap tests are compromised by the presence of contaminants such as mucous, blood cells and obscuring inflammatory cells. These contaminants lead to false negative results, false positive results, and a significant amount of follow-up work. See, e.g., Koss, L. G. (1989), The Papanicolaou Test for Cervical Cancer Detection: A Triumph and a Tragedy, JAMA 261:737-743; see also DeMay, “Problems in Pap Smear Interpretation”, Arch. Pathol. Lab. Med. 121:229-23 (1997).
In view of the above, there is a need for complementary molecular diagnostic methods for the analysis of cells that are present in a liquid medium containing a fixative. Such methods are not straightforward, however, because it is not always possible to perform such methods on fixed cells. For example, certain fixatives (e.g., those transport media employed in THINPREP™ or SUREPATH™ test systems) may cause particular cellular proteins to precipitate or aggregate, thereby making those proteins insoluble and difficult or impossible to reliably detect using conventional means, e.g., using an enzyme-linked immunosorbancy assay (ELISA) or another immunological test.
There is therefore a great need for methods and compositions for extracting proteins from fixed and unfixed cells in a manner that allows them to be suitable for use in molecular, e.g., immunological, detection assays. The invention described herein meets this need, and others.